1. Field of the Invention
This disclosure relates generally to energy storage devices (e.g., batteries), and in particular, to a system for monitoring and controlling the discharge cycle of the cells in a battery.
2. Description of Related Art
Circuits are known in the art for monitoring the condition of batteries and their cells. Such known circuits measure the voltage, the current and discharge cycle of the batteries and use this information to determine the status of the battery as a whole, or any particular battery cell.
In multi-cell batteries, a low-capacity cell will typically discharge more rapidly than the other cells, causing that particular cell to "reverse charge". Once a cell becomes reverse charged (i.e., forced below 0 volts by adjacent, electrically connected cells, during a discharge cycle), the cell becomes permanently damaged and will develop poor recharging characteristics. In prior art batteries, once a cell becomes reverse charged, the damaged cell will affect the operating characteristics of the entire battery. The damaged battery will have lower capacity and will become discharged more rapidly than a healthy battery.
It would be desirable to manufacture a battery with cells that are "matched", having similar charging and discharging characteristics. Although, a battery mate up of matched cells will typically outperform unmatched batteries, the cell-matching process is difficult to implement cost effectively and is rarely performed, most commercially available batteries are made from unmatched cells and therefore perform somewhat unpredictably. Furthermore, in such matched-cell batteries, the charging and discharging characteristics of each cell will change differently each time the battery is charged and discharged so that over a period of time, such matched cells will invariably become unmatched.
In an unmatched battery, each cell will operate with its own particular charging and discharging characteristics, and one of the cells wig inherently have poorer operating characteristics (e.g., will not hold a high charge and will rapidly discharge) than the others. This low-capacity cell will typically be the first one to discharge as the battery supplies power to a connected load. Once the low-capacity cell becomes fully discharged, it will be drawn ino a reverse charge condition and will become damaged. The effective output of the battery as a whole is dictated by the effective output of the low-capacity cell. When the low-capacity cell becomes discharged, the outlet voltage of the remaining "good" cells is reduced by the reversed charged voltage of the low-capacity cell and the battery, as a whole, rapidly fails.
Devices in the prior art are capable of detecting failing cells and responding to protect the remaining cells of a battery. For example, U.S. Pat. No. 5,258,244 measures voltage differences across individual cells using internal impedances of each cell. Failing cells, as determined by an increase in their internal impedance, may the be isolated from the other cells. U.S. Pat. No. 4,871,956 monitors the condition of cells by sequentially sampling the voltage of each cell and comparing the sampled cell voltage with a reference voltage to generate voltage differences which are stored in a shift register for each cell. If the voltage difference is sufficiently high, the cell is isolated from the other cells.
The selective identification and isolation of discharging cells in the prior art has distinguished defective cells from healthy cells, and so has not treated both defective and healthy cells for common monitoring and protection.
It is a first object of the invention to provide a battery monitoring/controlling circuit that monitors each cell of a battery and isolates any cell whose charge reaches a predetermined threshold voltage without affecting the operation of any adjacent cell.
It is another object of the invention to provide such a battery monitoring/controlling which overcomes the deficiencies of the prior art.
It is another object of the invention to provide such a battery monitoring/controlling circuit that allows each cell of a battery to operate independent of the other cells.